1. Field of the Invention
The present invention relates to a surface acoustic wave device and a communication apparatus, such as a cellular phone, employing the same.
2. Description of the Related Art
Surface acoustic wave devices incorporating SAW (Surface Acoustic Wave) filters have been used in a variety of communication apparatuses.
As the modern communication apparatuses have become more compact, adapted for higher-frequency operations, and more functional, there arises an increasing demand for widening the pass bandwidth of the SAW filter. For instance, a high-performance wide-band filter having an effective pass bandwidth of not less than 80 MHz (a fractional bandwidth of about 4% or more) is desired as a filter for use in a cellular phone operating in a transmission band of 1.9 GHz. It is noted here that the “fractional bandwidth” means a value given by dividing the effective pass bandwidth by the center frequency of the passband.
For achieving such a wide-band transmission, there has been proposed a double-mode SAW resonator filter which includes three IDT (Inter Digital Transducer) electrodes formed on a piezoelectric substrate and utilizes a first-order and third-order longitudinal modes.
FIG. 22 is a plan view showing an electrode configuration of a conventional resonator-type SAW filter. IDT electrodes 202, 203, 204 arranged on a piezoelectric substrate 201 each comprise a pair of comb electrodes opposing to each other. A surface acoustic wave is generated by applying an electric field to the pair of comb electrodes. A signal is inputted to the central IDT electrode 203 via an input terminal 210 connected to one of the comb electrodes thereof, thereby causing propagation of an excited surface acoustic wave to the IDT electrodes 202, 204 disposed on the opposite sides of the IDT electrode 203. The signal is transmitted from a respective one of the comb electrodes constituting the individual IDT electrodes 202, 204 to an output terminal 213 via IDT electrodes 206, 208 and an IDT electrode 207, so that the output terminal provides an output. Thus, the resonator-type electrode patterns may be interconnected in two-stage cascade connection fashion so as to achieve an increased amount of out-of-band attenuation as filter characteristics. In the figure, reference numerals 205, 209 each represent a reflector, whereas a reference numeral 211 represents a ground terminal.
Likewise to FIG. 22, FIG. 23 is a plan view showing an electrode configuration of a conventional SAW filter. The filter includes IDT electrodes 102, 103, 104 and a reflector 107 formed on a piezoelectric substrate 101. In the figure, reference numerals 105, 106 each represent an inter-electrode portion defined between adjoining IDT electrodes, a reference numeral 108 representing an inter-electrode portion defined between the IDT electrode 103, 104 and the reflector 107, a reference numeral 111 representing an input terminal, a reference numeral 112 representing a ground terminal, a reference numeral 113 representing an output terminal.
The surface acoustic wave is excited by inputting a signal to the input terminal 111 connected to the IDT electrode 102. The surface acoustic wave is propagated to the IDT electrodes 103, 104 on the opposite sides of the IDT electrode 102, so that the signal is outputted from the output terminal 113 connected to the IDT electrodes 103, 104. The surface acoustic wave is reflected by the reflectors 107 on the opposite ends of the filter, thereby forming a standing wave.
The mode of the standing wave includes a first-order mode and a higher-order mode (third-order mode) because of the presence of the three IDT electrodes 102, 103, 104. The pass characteristics of the filter depend upon resonances generated in these modes. Hence, the filter may be broadened in the passband by controlling a distance between the resonant frequencies occurring in these modes.
Conventionally, the distance between the resonant frequencies is controlled as follows. While all the IDT electrodes 102, 103, 104 are so configured as to have the same center-to-center distance (pitch L) between respective pairs of adjoining electrode fingers, the distance between the resonant frequencies is controlled by controlling a distance ‘d’ between adjoining IDT electrodes.
According to the aforementioned control method, the conventional double-mode SAW resonator filter employing a LiTaO3 substrate as the piezoelectric substrate thereof can achieve a fractional bandwidth of about 0.40% (see, Japanese Unexamined Patent Publication No. 1-231417) or, at most, of about 2% (see, Japanese Unexamined Patent Publication No. 4-40705). Although the conventional filter has achieved the maximum fractional bandwidth of 3.7% (see, Japanese Unexamined Patent Publication No. 7-58581), temperature variations must be taken into consideration when the filter is in actual operations. Furthermore, the filter is prone to frequency variations due to the variations in the configuration of the formed electrodes. Accordingly, it is impracticable to apply the conventional double-mode SAW filter to the communication apparatuses, such as the cellular phones, which require a broad pass bandwidth.
In addition, the following attempt has been made to broaden the passband and to reduce the insertion loss. A narrow electrode-finger pitch portion (the inter-electrode portion 105, 106 between the adjoining IDT electrodes shown in FIG. 23) is provided between the respective ends of the adjoining IDT electrodes, thereby reducing radiation loss into a bulk wave, the loss occurring at place between the IDT electrodes. In this state, a resonant mode is conditioned such that the filter may achieve a broader pass bandwidth and a reduced insertion loss (see, Japanese Unexamined Patent Publication No. 2002-009587 and Published Japanese Translation of PCT International Publication for Patent Application No. 2002-528987).
However, the piezoelectric substrate encounters a pyroelectric effect caused by temperature fluctuations during the fabrication process, so that the electric charges concentrate on the narrow pitch portion between the IDT electrodes. The narrow pitch portion may encounter electric discharge caused by potential difference between the electrode fingers and hence, the IDT electrode may be damaged.
It is an object of the invention to provide a surface acoustic wave device having excellent filter characteristics featured by a low insertion loss and a broad passband, as well as to provide a communication apparatus using the same.